Many industrial processes involve the interaction of liquids with solid surfaces. Often, it is desirable to control or influence the manner of the interaction, particularly the degree of wetting of the surface, so as to achieve a specific result. For example, surfactants are sometimes added to liquids used in cleaning processes so as to achieve greater surface wetting. In a converse example, liquid repellent coatings are sometimes added to clothing products to reduce surface wetting and accelerate drying of the clothing.
Efforts have been underway for decades to analyze and understand the principles and properties affecting surface wetting. There has been a particular interest in liquid “phobic” surfaces, which are surfaces that are resistant to wetting by liquids. Such surfaces may be referred to as hydrophobic where the liquid is water, and lyophobic relative to other liquids. If the surface resists wetting to an extent that a small droplet of water or other liquid exhibits a very high stationary contact angle with the surface (greater than about 120 degrees), if the surface exhibits a markedly reduced propensity to retain liquid droplets, or if a liquid-gas-solid interface exists at the surface when completely submerged in liquid, the surface may be generally referred to as an ultrahydrophobic or ultralyophobic surface. For the purposes of this application, the term ultraphobic is used to refer generally to both ultrahydrophobic and ultralyophobic surfaces.
Ultraphobic surfaces are of special interest in commercial and industrial applications for a number of reasons. In nearly any process where a liquid must be dried from a surface, significant efficiencies result if the surface sheds the liquid without heating or extensive drying time.
Moreover, friction between the liquid and the surface is dramatically lower for an ultraphobic surface as opposed to a conventional surface. As a result, ultraphobic surfaces are extremely desirable for reducing surface friction and increasing flow in a myriad of hydraulic and hydrodynamic applications on a macro scale, and especially in microfluidic applications.
It is now well known that surface roughness has a significant effect on the degree of surface wetting. It has been generally observed that, under some circumstances, roughness can cause liquid to adhere more strongly to the surface than to a corresponding smooth surface. Under other circumstances, however, roughness may cause the liquid to adhere less strongly to the rough surface than the smooth surface. In some circumstances, the surface may be ultraphobic.
Efforts have been made previously at introducing intentional roughness on a surface to produce an ultraphobic surface. The roughened surface generally takes the form of a substrate member with a multiplicity of microscale to nanoscale projections or cavities, referred to herein as “asperities”.
Previous attempts at producing ultraphobic surfaces with micro/nanoscale asperities have been only partially successful. Generally, while the prior art surfaces have exhibited ultraphobic properties under some circumstances relative to liquid droplets carefully placed on the surface, the properties generally disappear when a droplet is impacted with the surface.
Moreover, fluid pressure in many industrial applications where ultraphobic surfaces are desirably used often exceeds one atmosphere, and in extreme applications, may reach hundreds of atmospheres. Ultraphobic surfaces produced to date appear to be effective as an ultraphobic surface only up to about 0.1 atmospheres.
Prior art ultraphobic surfaces are often formed with delicate polymer or chemical coatings deposited on the substrate. These coatings are easily physically damaged so as to be ineffective.
There is still a need in the industry for a durable ultraphobic surface that retains ultraphobic properties when impacted by liquid or under a column of liquid at pressure heads exceeding at least one atmosphere.